July 6, 2007 — NASA is preparing to dispatch an innovative robotic probe to the asteroid belt, an area of the solar system between Mars and Jupiter riddled with bodies from the formation of the solar system.
Judging from the debris, the solar system's birth was not an easy one. As the primordial disc of gas and dust collapsed under its own gravity, individual bodies began to form, with the rocky ones gathering closer to the sun and the gaseous giant planets migrating outward.
The area between the two groups became the fallout zone, a junkyard littered with fetal planets destroyed by impacts. The science probe Dawn, which is scheduled for launch Monday, is humanity's first attempt to take a closer look at the remains.
"I think many people mistakenly think of asteroids that we're going to as little chips of rock. We're going to worlds," said Dawn lead engineer Marc Rayman, with NASA's Jet Propulsion Laboratory in Pasadena, Calif.
Vesta's Secrets
Dawn's first target is Vesta, the fourth-largest asteroid in the region and the only object visible from Earth with the naked eye. It has an iron core and signs of lava flows on its surface. Scientists believe it is very similar to Earth's moon.
Vesta also has a gaping crater, so deep it exposes the asteroid's mantle. Dawn is expected to help answer questions about Vesta's composition and how it formed. The spacecraft has three science instruments to study surface features and determine chemical composition.
"From that we try to work back to how the whole thing was put together and what happened to it," said lead scientist Chris Russell, with the University of California at Los Angeles.
Researchers also plan to measure the strength of Vesta's gravity field to more precisely determine the body's mass and density.
After six months of observations, Dawn will do what no spacecraft has done before: leave orbit and settle around a second target.
Typically, spacecraft do not have the tremendous amounts of fuel that would be necessary for multi-target maneuvers. But Dawn is equipped with three fuel-efficient electric ion thrusters, which can be turned on and off as needed. The system works by sending an electrical charge through xenon fuel, leaving a stream of highly charged ions.
It takes a long time to build up speed — on Earth, Dawn would need four days to accelerate from 0 to 60 mph — but once momentum builds in the vacuum of space, the probe will become the fastest man-made object to fly through the solar system.
The engine was tested on a highly successful prototype spacecraft called Deep Space 1, which operated from 1998 to 2001.
"The ions come out of the engine much, much faster than if you would burn some chemical in the rocket, and that allows us to carry a smaller amount of fuel, or to get much greater efficiency out of the fuel that we are carrying," Russell said.
Round Two
Dawn's second target is the dwarf planet Ceres, the king of the asteroid belt. Though relatively close to Vesta, icy Ceres formed under vastly different circumstances. It contains water-bearing minerals and possibly a weak atmosphere.
"It's a frustrated planet in the sense that it would have continued to accrete into a large body had Jupiter not moved into the neighborhood and started to scoop up all the material," said JPL's Carol Raymond, the deputy principal investigator. "What we have is a little capsule of information about what the conditions were in the initial dust cloud of the primordial solar system."
